1. Field of the Invention
This invention relates to a method, system and materials for use in hydrogen gettering in conjunction with microelectronic and microwave components that are generally hermetically sealed in an enclosure typically referred to as a xe2x80x9cpackagexe2x80x9d.
2. Brief Description of the Prior Art
The effect of molecular and atomic hydrogen is of particular concern for hermetically sealed microelectronic devices, generally those containing group III-V semiconductor materials and specifically gallium arsenide microwave integrated circuits. It is known that these devices degrade as a function of time and temperature in the presence of even small amounts of molecular and atomic hydrogen. The amount of hydrogen which is tolerable depends upon the end use of the device and the required life expectancy thereof. In general, the presence of hydrogen in amounts of about 10 parts per million or less are tolerable but not easily obtainable. The mechanism under which this degradation occurs has been hypothesized by several sources, however there is no agreement at present as to the mechanism causing such degradation.
It is known that prior proposals to alleviate this problem have been made using organic hydrogen getter materials or inorganic getter materials in conjunction with an organic binder. The purpose of the getter is to capture as much as possible of the free gaseous hydrogen within the package interior, consequently decreasing or eliminating the rate of degradation of the hydrogen degradable material within the package, specifically gallium arsenide microwave integrated circuits and/or other hydrogen sensitive devices. The organic materials are generally unsatisfactory because they are: 1) limited in temperature range and/or 2) are reversible such that previously captured hydrogen may be released at elevated temperatures or with time and/or 3) because the getter rate decreases with time as a function temperature, ultimately limiting the temperature and/or length of time that the getter will function. It follows that other empirical approaches are required to minimize the above described problem.
In accordance with the present invention, the approaches taken to minimize the problem as discussed above involve the use of a getter of several different types and formed in several diverse manners.
One such getter involves the application of a transition metal, preferably nickel, palladium or platinum, using physical vapor deposition or electroplating. The deposited transition metal can be used in the pure metallic state or prepared as an oxide of the metal. In the pure form, transition metals such as platinum, palladium and nickel are known to absorb significant amounts of hydrogen without appreciable increase in the volume of the metal. In addition, the hydrogen can react with the pure metal to form hydrides and further getter the hydrogen. Also, the transition metals such as nickel, palladium and platinum can be oxidized to form a transition metal oxide through oxidation of the metal surface. In operation, the transition metal oxide reacts with hydrogen to form the elemental metal and water. A water getter can be included in the package to prevent accumulation of excessive levels of water vapor. The metal can be oxidized by a number of techniques, including thermal oxidation, plasma oxidation or chemical anodization. An alternate method of preparing the metal oxide is by direct sputter deposition of the metal oxide or by using reactive sputtering of the metal in an oxygen environment. The stoichiometry of the oxide is controlled to select the desired oxide type (e.g., PdO), such as, for example, through control of the reactive sputter deposition parameters.
The getter application location and/or size for both metal and metal oxide area are controlled by physical deposition masks, selective plating using plating masks or selective etching of the metal or metal oxide. Etching techniques include chemical or physical techniques. The getter is applied anywhere in the package interior and sized to assure any hydrogen within the package interior is completely captured. This approach has many advantages in that a very thin layer of metal or metal oxide can be formed, adding negligible weight or thickness to the package. Further, the described inorganic getters do not suffer from the temperature limitations of organic getters. In addition, a number of transition metals and/or metal oxides including those referenced above react with hydrogen over a wide temperature range and irreversibly getter hydrogen.
A specific approach to the problem is to utilize titanium as a housing or lid with or without a hydrogen permeable coating on the titanium. A desired hydrogen permeable coating is palladium which can be plated or vacuum deposited on top of the titanium. The purpose of the palladium is to prevent the titanium from oxidizing or otherwise being hydrogen getter inhibited. The titanium can also be used in bulk or sheet form as a discrete component which is adhesively or otherwise attached within the module. The surface area of the titanium can be substantially increased by forming a porous titanium structure, thus substantially increasing the rate of hydrogen gettering and decreasing the weight of the getter.
A second approach is to sputter, ion beam or otherwise vacuum coat a lid and/or housing with titanium to form a getter. The titanium can be overcoated in the vacuum deposition with a hydrogen permeable membrane such as palladium. This approach offers a very light weight getter with high surface area and excellent adhesion to metal oxides due to the high Gibbs free energy of oxide formation of titanium.
A third approach is to sputter deposit zirconium-vanadium-iron alloys with or without addition of one or more oxides of rare earth metals. These materials have shown hydrogen gettering characteristics in bulk form. The sputter deposited approach offers the unique advantages of light weight within a package, high surface area and good adhesion.
A fourth approach is to mix powders of titanium, palladium coated titanium or zirconium-vanadium-iron in an organic vehicle and screen print or otherwise dispense the material within a packaged module.
A fifth approach is to plate a hydrogen permeable but water impermeable membrane such as palladium onto a lid or other location in a module and etch or otherwise machine the support structure beneath the palladium to form a hydrogen permeable vent to the exterior of the package. This approach can also be incorporated into a hydrogen permeable plug or feedthrough that is soldered or welded to a housing or lid.
A sixth approach is to sputter deposit onto the interior surface of the package or module zirconium-vanadium-iron alloys with or without the additions of oxides of rare earth metals.
A further approach is to provide any hydrogen getter material including those named above which can be prepared in particulate or powder form. In application, the hydrogen and/or water getter material is ground or otherwise formed into a particulate or powder such that the particulate getter is mixed with an organic binder and screen printed on the desired surface. The binder is selected such that, upon screen printing and curing, the binder assures adhesive attachment of the getter to the desired surface and the binder allows permeability of the hydrogen or water through the binder and to the getter material. Typical binders that can be used include silicones, epoxies, acrylics, urethanes, polyimides and benzocyclobutene. A further benefit of this method of application is that the surface area of the getter in the mixed binder can be increased and controlled by varying the getter particulate size and thickness of the screen printed material. The screen printed method eliminates the need for forming the getter in bulk form, patterning or sizing it and adhesively attaching it within the hermetically sealed package in three separate operations. By the present technique, the hydrogen and or water getter is premixed with the binder and patterned, and adhesively attached in one operation, eliminating the separate, bulk material formation, sizing or patterning and adhesive attachment operations.
The screen printing method of getter attachment can also be extended to include an RF absorber material mixed with the hydrogen and/or water getter. The benefit of this method of application is that radio frequency absorber materials, including binders, commonly used today, release hydrogen in the curing process and after the curing process when installed in a package. By incorporating the hydrogen and/or water getter with the RF absorbing material in the presence of a binder, the additional absorber attachment process is eliminated. In addition, the getter is in intimate contact with the hydrogen releasing absorber material and will provide a more effective getter.
A still further method relates to the attachment of the getter to the interior of the microelectronic or microwave hermetically sealed enclosure using a numerically controlled placement and pump-dispensing system. Using this method, the inorganic getter is reduced to the desired particulate size and mixed with an epoxy or other organic binder and dispensed with precise thickness and dimensional placement control within the package interior. The RF absorber material can similarly be applied by itself or premixed with the hydrogen and/or water getter. Using this method of dispensing and application of the getter and/or radio frequency absorber, the pre-mixed material in paste form can also be used to attach components, eliminating multiple operations and combining component attachment, hydrogen getter attachment, water getter attachment and radio frequency absorber attachment or any combination thereof in a single operation with resultant savings in assembly time and savings of physical space within the interior of the module.